The present invention relates to semiconductor devices including nitride semiconductor layers and methods for manufacturing the semiconductor devices.
Field-effect transistors including nitride semiconductor layers such as GaN layers are expected to be widely used as electric power control elements because they have high withstand voltages and low resistivities. It is required that these electric power control elements are normally-off type elements. However, it is difficult to make field-effect transistors including nitride semiconductor layers normally-off type transistors.
International Publication WO 03/071607 discloses a transistor that uses two-dimensional electron gas accumulated at the interface, that is, at the hetero junction between an AlGaN layer and a GaN layer as carriers. This transistor is configured in a way that there is no hetero junction under the gate electrode of the transistor to prevent the two-dimensional electron gas from generating under the gate electrode. International Publication WO 03/074607 states that a normally-off transistor can be realized with the use of this configuration.
Japanese Unexamined Patent Application Publication No. 2011-044647 discloses a transistor that has a configuration that is formed as follows. First, a first nitride semiconductor layer, a second nitride semiconductor layer, and a third nitride semiconductor layer are stacked, and two-dimensional electron gas is generated at the interface between the second and third nitride semiconductor layers. Next, a concave portion, which reaches the first nitride semiconductor layer, is formed in the third nitride semiconductor layer and the second nitride semiconductor layer, and a gate insulating film and a gate electrode are embedded in this concave portion. In Japanese Unexamined Patent Application Publication No. 2011-044647, the concave portion penetrates the second nitride semiconductor layer. In addition, Japanese Unexamined Patent Application Publication No. 2011-044647 discloses that the second nitride semiconductor layer is formed by stacking plural AlGaN layers each of which has an Al concentration different from each other.